This invention is directed to palladium-based alloys which consist essentially of palladium, nickel and silicon and are particularly useful for brazing stainless steels employed in orthodontic devices at low brazing temperatures. More particularly, the invention is directed to palladium alloys consisting essentially of about 3 to about 30 atom percent nickel, about 15 to about 20 atom percent silicon and the balance being essentially palladium plus incidental impurities. The alloys of the present invention have a liquidus temperatures of less than about 880.degree. C.
Brazing is a process of joining metal parts, often of dissimilar composition, to each other. Typically, a brazing filler metal that has a melting point lower than that of the parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong, preferably corrosion resistant, joint is formed.
Brazing filler metals comprising nickel palladium alloys have been developed which exhibit good corrosion and erosion resistance. Such alloys which have been disclosed, for example, in U.S. Pat. Nos. 4,508,257, 4,405,391 and 4,448,618. Among the alloys disclosed in the prior art, are the ternary nickel-paladium-silicon alloys disclosed in U.S. Pat. No. 4,508,257. These alloys have large concentrations of nickel and silicon and exhibit rather low liquidus temperatures. More specifically, these alloys have liquidus temperatures in the range of about 877.degree.-948.degree. C.
More recently, a new class of nickel palladium alloys was developed which are also substantially free of boron. These alloys, disclosed in U.S. Pat. No. 4,746,375, overcome problems associated with brazing cemented carbide parts which require brazing at low temperatures (less than about 950.degree. C.).
Base parts of metallic orthodontic devices are comprised of thin, approximately 100 m mesh and slotted posts both of which are usually made from SAE 300 series stainless steels and are joined together by brazing. Currently, gold and silver-based alloys in either wire or powder/paste form are used as brazing filler metals in joining together these parts. The conventional gold-base alloy most frequently used is 82/18 Au--Ni (weight percent). This gold-base alloy suffers from the following deficiencies: it is very expensive to produce; it has moderate strength: it melts at above about 950.degree. C. resulting in detrimental annealing of the thin stainless base metal parts; and, it produces joints having a yellowish color. For orthodontic devices, it is particular important that the color of the joint not contrast with the color of the stainless steel base metal parts. The result is an unfavorable appearance.
Alternatively, silver-base alloys, which are mostly used in the paste form, have been employed to braze orthodontic devices. However, silver-based alloys are prone to the formation of joint porosity which impairs joint strength and can result in joint-pit corrosion and oxidation. Moreover, the color of the joints also contrasts with the color of the stainless steel device. Further, usage of the paste form necessitates expensive cleaning of the brazed parts to remove the slag formed as a result of the decomposition of the organic binder used in the production of the paste employed as the brazing material.
An attempt was made to address the problems associated with gold and silver based brazing alloys using alloys disclosed in U.S. Pat. No. 4,508,257. Specifically, an alloy designated as MBF-1005 METGLAS.RTM. brazing foil (Allied-Signal Inc.), having a composition Ni.sub.bal Pd.sub.30 Si.sub.15 has been used. This alloy has a liquidus temperature in the range of about 830.degree. to 860.degree., which is well below that of the gold-nickel alloys of the prior art. It forms a strong joint and has a color similar to the stainless steel. However, a limitation using this nickel-base foils is the ability to cast the foil into a ductile shape thicker than 3-4 mils (approximately 75-100 .mu.m).
Accordingly, there is a need in the art for brazing filler materials suitable for brazing thin stainless steel parts employed in orthodontic devices which will produce high strength joint, acceptable appearance, can be used with brazing temperatures less than 900.degree. C., and can be produced as a thick, ductile brazing product.